Chapter 29 

 THE TELEMETERING OF HUMAN SUBJECTS AND ANIMALS UNDER WATER 



Samuel Bellet, Allan Slater, and David Kilpatrick 



Philadelphia General Hospital 



Philadelphia, Pennsylvania 



INTRODUCTION 



To obtain information relative to the interaction of cardiac response to under water work- 

 ing tasks and swimming, acoustic underwater telemetering of EKG* signals from the swimmer 

 to the habitat and hard wire transmission to the staging vessel was experimentally performed 

 in Sealab II operations. Success was achieved in obtaining a limited number of electrocardio- 

 grams. A number of technical difficulties were revealed, as well as possible solutions thereto. 



Ultrasonic telemetry was selected as the method for transmitting information from the 

 swimmer to the habitat. After studying a number of techniques of modulating an ultrasonic 

 carrier, frequency modulation was chosen as the best method to transmit the data in the trans- 

 mission medium (sea water). The factors considered were bandwidth, medium inhomogeneity 

 effects, multipath propagation, and battery energy utilization. 



The design of a sonic telemetry system is a relatively simple problem. The signal band- 

 width of a few hundred cycles per second for cardiac electrical activity, for example, is so 

 small that frequency modulation is compatible with the medium. The primary limitation of 

 water as a communication medium is carrier bandwidth. This is due to the rapid increase of 

 propagation attenuation with increasing frequency. A typical submarine voice communication 

 system must heterodyne the voice signal to a higher but still audio frequency, and "transmit" 

 single sideband in order to limit the carrier bandwidth to the same value as the signal, or 

 about 3,000 cps. It is practical to utilize frequency modulation with a total deviation of less 

 than 3,000 cps for a cardiac signal of 100 cps bandwidth. Frequency modulation has many ad- 

 vantages for the accurate transmission of information in a noisy medium. 



BASIC SYSTEM 



The complete data link can be broken down into four subsystems (Fig. 79). Electrodes 

 pick up electrical signals induced by the heart in the chest wall. These signals are relayed to 

 the transmitter, where they are converted to a frequency- modulated acoustical wave. This 

 wave is propagated from the diver through the sea to the receiver and is processed to recover 

 electrocardiographic information. Output from the receiver is recorded with a Sanborn re- 

 corder located in the Berkone staging vessel. 



Electrode Details 



Two types of electrodes have been used. All tests prior to delivery of the equipment to 

 Sealab II used Beckman electrodes, while tests at Sealab n used both silver-silver chloride 

 (Beckman) and tin (Sem-Jacobsen) electrodes. Electrode construction is shown in Fig. 80. 



* Electrocardiogram. 



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